1. Field of the Invention
The present invention generally relates to image scanners and, more particularly, is directed to a method and apparatus for adjusting a dynamic range of an output signal of a solid state image sensor.
2. Description of the Related Art
In an image scanner, a light source illuminates an original document and a one-dimensional image sensor (line sensor) moves over the original document in a direction perpendicular to the length direction of the image sensor to read the original document. The light source might be a special fluorescent lamp of high brightness and the image sensor might be a one-dimensional CCD (charge-coupled device) image sensor.
Incidentally, the dynamic range of the image sensor is determined by a ratio between the maximum signal level and the minimum detected signal level. The maximum signal level is determined by the structure of the image sensor and the quantity of incident light, whereas the minimum detected signal level is influenced by various noise components generated within the image sensor. As described above, the dynamic range of the CCD image sensor has an inherent predetermined range, but the fluorescent lamp used as the light source is gradually reduced in quantity of light with a time period, resulting in the quantity of light incident on the CCD image sensor being reduced. If the quantity of light incident on the CCD image sensor is reduced, then the maximum signal level also is reduced, which unavoidably lowers the dynamic range and which increases the noise component. As a consequence, the output signal of the CCD image sensor is deteriorated in S/N (signal-to-noise) ratio.
Accordingly, if the image scanner is continuously utilized over a long period of time continuously, then the dynamic range of the image sensor (i.e., dynamic range of the output signal of the image sensor) is changed so that the image scanner cannot read the image under satisfactory conditions such as when the S/N ratio is small.
Therefore, in the image scanner, the following methods are proposed in order to adjust the dynamic range of the image sensor:
(1) To provide an AGC (automatic gain control) circuit;
(2) To control storage time of the image sensor; and
(3) To adjust the light source.
In accordance with the proposed method (1), the AGC circuit is interposed between the image sensor and an analog-to-digital (A/D) converter to thereby control the level of the signal supplied to the A/D converter so as to fall in a proper range.
According to the proposed method (2), by changing the frequency of the driving clock of the image sensor, the storage time of the image sensor is changed, thereby the dynamic range of the image sensor being adjusted.
According to the proposed method (3), the supply of voltage to the light source is turned on and off and a on-off duty ratio or on-off cycle is changed, thereby the quantity of light of the light source being adjusted.
The above methods (1)-(3), however, cannot eliminate the following shortcomings and disadvantages. That is, according to the method (1), since the dynamic range of the image sensor is limited it is frequently observed that the image sensor cannot cope with the change of the quantity of light of the light source. Also, in the CCD image sensor, while the amount of the signal charge is reduced in proportion to the quantity of light, the amount of the noise component is reduced in proportion to the square root of the charge amount so that, when the quantity of light of the light source is reduced, then the ratio of noise contained in the output signal of the image sensor is increased. Accordingly, although a signal of proper level is supplied to the A/D converter, the signal supplied to the A/D converter is deteriorated in S/N ratio.
According to the method (2), since the hardware is limited, the frequency of the driving clock cannot be increased so much. Also, since the interface to the external devices is limited, the clock frequency cannot be decreased so much. In addition, if the clock frequency is decreased, then a dark current of the image sensor is increased, which leads to the deterioration of S/N ratio of the output signal of the image sensor.
Furthermore, according to the method (3), because a flicker occurs when the quantity of light of the light source is reduced too much, there is then a limit to reduce the quantity of light. Also, since the light source is frequently turned on and off in order to adjust the quantity of light, the life of the light source is shortened and the light source becomes unable to provide sufficient quantity of light earlier, thereby the dynamic range of the image scanner being lowered.